Now It's Your Turn: Self-Study Guide

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Introduction

Period of Rotation Examples

Angular Velocity Examples

Strategy

Description:

Period of Rotation

The period of rotation of a Solar System object is the length of time it takes that object to spin once around on its axis. For example the Earth takes 24 hours to spin once around its axis. Its period of rotation is 24 hours or a day.

First, let's consider how to find the period of rotation of an object, be it a planet, orange, or basketball.

One way is to find a landmark or surface feature, and to measure the
time it takes for an object to spin around and come back to the same spot. (Unfortunately, not all planets have a mark
"stamped" on them. You'll have to find something to use as a landmark or surface feature later on.)

Before you start exploring with real planets, let's practice with something more simple, such as basketball.

Examples:

If a planet turns halfway (180 degrees) in 12 hours, how long will it take to go all the way around (360 degrees)?

Often you can't see the planet during its entire rotation. (The Sun comes up in the morning and obscures our view of the planets; the planet slips behind clouds or below the horizon; etc.)
Let's consider the following example.

We know that a surface feature of a rotating object covers 20 degrees in 4 hours. What is the period of its rotation? (To solve this problem, you will need to use some ratios.)

Looks like you now understand what the period of rotation is, and are ready to work with rotation rates (or angular velocities, as they are sometimes called.)

Angular Velocity

We are approaching our GOAL - calculating the rotation rates (or angular velocity) of some planets.Remember, angular velocity is the angle by which an object turns in a certain time. Angular velocity can be expressed in degrees per unit time (second, minute, hour, etc.), radians per unit time, or even revolutions per unit time.

But first let's practice with some simple examples again. We will begin with racing cars.

Examples:

A circle has 360 degrees, right? So an Indycar driver speeds through 360 degrees every time she makes a lap (we'll use a circular race track). If the racer makes 360 degrees (one loop) in one minute, how many degrees per second does she cover?

Solution

To find the rotation rate, or angular velocity, you will need to divide the number of degrees by the number of seconds in one minute. The answer is: (360 degrees) / (60 seconds) = 6 degree/second

This problem is not about cars, it is about planets. Assume that a planetary feature moves 36 degrees in 4 hours. Use the angular velocity to find how long it takes for that feature to go all the way around the planet.